The term “rigging” (sometimes referred to as industrial rigging or field rigging) is the branch of securing heavy loads in order to prepare the load to be lifted, moved or transported. Rigging usually refers to the ropes, wires, slings, and chains used to secure the load.
Wire rope slings made of a plurality of metal strands twisted together and secured by large metal sleeves or collars are known in the industry. Since wire rope slings are made of metal, they typically do not require external protection that may be afforded by a covering material. During the recent past, industrial metal slings have seen improvements in flexibility and strength. Metal slings are, however, relatively stiff, inflexible, heavy and subject to fatigue and corrosion when compared to non-metal or synthetic fiber slings.
Synthetic fiber slings have gained popularity in recent years and are replacing metal slings in many circumstances. Thousands of synthetic slings are used on a daily basis in a broad variety of heavy load lifting applications, ranging from ordinary construction (e.g., nuclear power plants, skyscrapers and bridges), plant and equipment operations, ship building (e.g., oil rigs), and the like.
An advantage of synthetic slings over metal slings is that they have a high load-lifting performance strength-to-weight ratio, providing for lighter, more flexible and stronger slings when compared to their heavier, stiff and bulkier metal counterparts. Synthetic slings may also be designed to have resistance to fatigue and corrosion based on the expected working environment of the sling through selection of particular materials for the synthetic sling. Another feature of synthetic slings is the encasement of the load bearing strands of the sling in a protective cover that protects the load-bearing strands from the working environment. The protective cover or sheath requires particular steps in the manufacturing process, primarily encasing the core strands or load-bearing strands inside the protective cover.
Synthetic slings are usually comprised of a lifting core made of twisted strands of synthetic fiber and an outer cover that protects the core. The most popular design of synthetic slings is a roundsling in which the lifting core forms a continuous loop and the sling is generally ring-shaped in appearance. The lifting core fibers of such roundslings may be derived from natural materials (e.g., cotton, linen, hemp, etc.), but are preferably constructed of synthetic materials, such as polyester, polyethylene, nylon, polypropylene, aramids, and the like. The outer covers of synthetic slings are also preferably constructed of synthetic materials and are designed to protect the core fibers from abrasion, cutting by sharp edges, or degradation from exposure to heat, cold, ultraviolet rays, corrosive chemicals, caustic gasses, or other environmental pollutants.
A method of manufacturing prior art roundslings is to twist a plurality of yarns together to form a single strand and the strand is rolled into an endless parallel loop that forms the core. In a separate step, the cover is manufactured as a flat piece and the lifting core is laid on the flat cover material. The flat cover material is subsequently bent around the endless core and the two longitudinally extending edges of the cover are sewn together, thereby encasing the core or lifting fibers. This method of manufacturing roundslings is time consuming and labor intensive, thus increasing the costs to manufacture the sling. Another prior art method involves mechanical wrapping of the core strands into a protective cover cut at one location along its length and subsequent closing of the cover at the cut. This prior art method is generally described in U.S. Pat. No. 7,568,333, which is incorporated herein by reference in its entirety.
The core strands, lifting fibers or lifting cores are tensioned during the manufacturing process to produce a sling wherein each of the core strands is generally, equally pre-loaded during production. It is preferred that each sling produced or constructed on a sling manufacturing machine has the same tension to produce consistent slings. Prior art sling tensioning has been substantially manually monitored and applied through the skill and experience of the operator.
These prior art methods of manufacturing roundslings are generally labor intensive, require physical exertion of the operator during various portions of the process and may result in inconsistency of tensioning from machine to machine and operator to operator. One of the labor intensive processes includes readjusting an idler roller of the system to ensure appropriate tension in the roundsling is maintained during the process. It is desirable to develop a system and method for reducing the physically intensive process of adjusting the idler roller and accurately maintaining desired tension in the roundsling during production.